1. Field of the Invention
This invention relates generally to a gear arrangement for transmitting torque through an angle, and more particularly to a gear arrangement using a pair of face gears as torque dividers to transmit power from a spur gear-type pinion through an angle.
2. Description of the Related Art
A variety of gear arrangements have been used for transmitting torque through an angle, or around a corner. For example, a gear arrangement employing a spur gear and a face gear is often used. A spur gear is a gear wheel with straight radial teeth on its circumference, the teeth edges being parallel to the axis of rotation. A face gear is a gear wheel having an axis of rotation perpendicular to that of the spur gear and having teeth on its face along the wheel periphery, the teeth being radially directed toward the center of the wheel. The spur gear acts as a pinion, driving or being driven by the larger face gear. Because the small spur gear has fewer teeth than the face gear, the spur gear turns with greater speed, but with less torque. The ratio between the number of teeth, resulting in the difference in speed between the gear wheels, is referred to as the speed reduction ratio.
Bevel gears are used to transmit torque through a 90.degree. angle. Bevel gears comprise gear wheels with straight teeth that mesh at an angle. Spiral bevel gears have curved teeth that also mesh at an angle and can transfer a greater amount of torque than a spur gear and face gear combination or straight bevel gears of comparable size. Spiral bevel gears, for example, are used in a typical automobile differential to transmit torque from a drive shaft to a rear axle. Spiral bevel gears are also used in conventional helicopter transmissions, where torque from the drive shaft of a longitudinally disposed engine must be turned through an angle of approximately 90.degree. to drive the rotor.
While spiral bevel gears are especially suited for transmitting torque through an angle, they do have drawbacks. Spiral bevel gears are extremely sensitive to changes in the relative location of the gears. Thus, spiral bevel gears are adversely effected by small amounts of thermal growth during operation and deflections of the gear supporting structure under load. The gears may not maintain optimum meshing through changes in temperature and deflections of the supporting structure. Because of this sensitivity to changes in the relative position of the gears, spiral bevel gears are more readily suited to low speed reduction ratios (ratios seldom greater than two to one) and are less suited to high speed reduction ratios (greater than four to one). The sensitivity to minute changes in relative position requires a heavy and complex support structure for spiral bevel gears. Such sensitivity also means that spiral bevel gears require complex manufacturing, assembly, and gear mating procedures.
In contrast, gear arrangements using a spur gear-type pinion and a face gear are much more forgiving of changes in relative size and position. The pinion can move in and out freely along its axis of rotation, within the limits of the length of its teeth, without negative effects on meshing with the face gear. Furthermore, a spur gear pinion has more tolerance for movement toward or away from its driven face gear than does an equivalent spiral bevel gear arrangement. Thus, gear arrangements using a spur gear and face gear are relatively unaffected by the thermal growth and deflection of the supporting structure experienced in many applications. For this reason, such gear arrangements easily accommodate speed reduction ratios greater than four to one. When high speed reduction ratios are required, such as for a helicopter drive, a two-stage transfer is often used. A spiral bevel gear stage is used to transfer the torque, ordinarily with less than a two-to-one transfer ratio, and a spur gear or planetary gear stage is used for the remaining speed reduction. While this is not as compact as a single stage, it does allow exploitation of the advantages of both types of gears.
In addition to sensitivity to thermal growth and deflections, spiral bevel gears do not exhibit conjugate action. A lack of conjugate action means that the velocity of the driven gear experiences minute acceleration and deceleration as the teeth go through meshing engagement, and manifests itself as objectionable noise and vibration. Spur/face gear combinations exhibit true conjugate action. Therefore, for a spur/face gear combination, the velocity of the driven gear remains constant as the teeth go through meshing engagement, resulting in relatively quiet and vibration free operation.
Thus, there is a need for a compact gear arrangement that can transmit large torque through an angle while accommodating thermal growth and deflection of the supporting structure, with reduced vibration and noise, and at high speed reduction ratios. Such a gear arrangement would provide the high torque transfer associated with spiral bevel gears and the conjugate action and tolerance of relative movement and thermal growth associated with conventional spur/face gear combinations.